This invention is concerned with photography and, more particularly, with image-receiving elements for use in photographic processes which are conducted outside of the camera in which the film is exposed.
U.S. Pat. No. 3,415,644 issued Dec. 10, 1958 to Edwin H. Land discloses photographic products and processes wherein a photosensitive element and an image-receiving element are maintained in fixed relationship prior to exposure, and this relationship is maintained as a laminate after processing and image formation. In these products and processes, the final image is viewed through a transparent (support) element against a reflection i.e., white, background. Photoexposure is made through said transparent element and application of the processing composition provides a layer of light-reflecting material to provide a white background for viewing the final image through said transparent support. The light-reflecting material (referred to in said patent as an "opacifying agent") is preferably titanium dioxide, and it also performs an opacifying function, i.e., it is effective to mask the developed silver halide emulsions so that the transfer image may be viewed without interference therefrom, and it also helps to protect the photoexposed silver halide emulsions from postexposure fogging by light passing through said transparent layer if the photoexposed film unit is removed from the camera before image-formation is completed.
U.S. Pat. No. 3,647,437, issued Mar. 7, 1972 to Edwin H. Land, discloses photographic products which may be processed outside of the camera in which the film is exposed, fogging of the film by ambient light being prevented by provision of one or more opacifying dyes, sometimes, referred to as light-absorbing optical filter agents, appropriately positioned in the film unit after photoexposure. In a particularly useful embodiment of that invention, the film unit is a film unit of the type described in the aforementioned U.S. Pat. No. 3,415,644 and comprises first and second sheet-like elements, the first sheet-like element comprising an opaque base carrying a silver halide emulsion, and the second sheet-like element comprising a transparent support carrying an image layer, i.e., a layer adapted to receive an imagewise distribution of an image-forming material initially present in said first sheet-like element. After photoexposure a processing composition, adapted to develop the exposed silver halide emulsion and to form the desired image in said image layer, is distributed in a thin layer between said sheet-like elements. The processing composition contains a light-reflecting pigment, such as titanium dioxide, and at least one light-absorbing optical filter agent, such as a pH-sensitive phthalein dye which is colored at the initial pH of said processing composition. As disclosed in said U.S. Pat. No. 3,647,437, the concentrations of said light-reflecting pigment and said optical filter agent(s) are such that the layer of processing composition is sufficiently opaque to light actinic to the silver halide emulsion that the film unit may be ejected from the camera immediately after the processing composition is distributed, notwithstanding the fact that the second sheet-like element will transmit light incident on the surface thereof. This opacification system is quite effective and is employed in Polaroid Land SX-70 film. The light-absorbing capacity of the optical filter agent is discharged after this ability is no longer needed, so that the optical filter agent need not be removed from the film unit. Where the optical filter agent is a pH-sensitive dye, such as phthalein indicator dye, it may be discharged or decolorized by reducing the pH after a predetermined time, e.g., by making available an acid-reacting material such as a polymeric acid.
In the preferred embodiments of the opacification system described in U.S. Pat. No. 3,647,437, the concentrations of the light-reflecting pigment and light-absorbing optical filter agent in the layer of processing composition will be such that that layer will have a transmission density of at least about 6 but a reflection density not greater than about 1. The presence of a long chain substituent, e.g., a long chain alkoxy group, on the optical filter agent is useful in reducing its diffusibility so that diffusion to the image-receiving layer is minimized.
A reflection density of about 1 will be recognized as very small compared with a transmission density of 6 or more for the same layer. In practice it has been possible to use a concentration of optical filter agents and titanium dioxide such that the reflection density of the processing composition layer, as measured about 30 seconds after distribution, is much lower than 1, e.g., about 0.5 t 0.6. While transferring dye and the emerging dye image may be seen at opacification system reflection densities of about 0.5, the presence of such temporary coloration of the highlight or white areas of the image, and the temporary distortion of the colors of the already transferring image dyes, is aesthetically undesirable.
As noted above, where the optical filter agent is a pH-sensitive dye, it is "discharged", i.e., rendered substantially colorless, by a reduction of the pH of the strata containing the optical filter agent. These strata include the light-reflecting pigment layer, provided by the processing composition, as well as the image-receiving layer and any other layers between the light-reflecting pigment layer and the transparent support through which the final image is viewed. This pH reduction is effected, to a pH level below the pKa of the optical filter agent, after a predetermined time. This delay is necessary in order that silver halide development be substantially completed before incident light is transmitted to the developing silver halide emulsions. Since the image dyes are preferably soluble and diffusible at the initial pH of the process but substantially nondiffusible at a lower pH, reduction of the pH to the appropriate lower pH after a predetermined period serves the very important function of controlling unwanted continued transfer of image dyes after the desired dye image has been formed.
It will be recognized that these desired results of pH reduction are only partly compatible, for early pH reduction to provide a white background early in the process could prematurely stop transfer of image dyes, resulting in a pale, i.e., low density, image which may also have an unbalanced color balance.
In U.S. Pat. No. 4,298,674 (issued Nov. 3, 1981 to E. H. Land, L. D. Cerankowski and N. Mattucci) and in U.S. Pat. No. 4,294,907 (issued Oct. 13, 1981 to I. Y. Bronstein-Bonte, E. P. Lindholm and L. D. Taylor), there is disclosed and claimed, diffusion transfer products and processes of the foregoing type where the background appears substantially white to the viewer, substantially immediately after the processing composition is applied while retaining opacification. As disclosed in said patents, it has been found that it is possible to significantly reduce the reflection density provided by the layer of processing composition containing the light-reflecting pigment and the optical filter agent without significantly reducing the transmission density thereof. This highly desirable improvement is obtained by decolorizing, substantially immediately after application of the processing composition, the optical filter agent immediately adjacent the interface between the processing composition and the layer of the second sheet-like element in contact with the processing composition. It is only necessary to decolorize a very small fraction of the applied optical filter agent in order to effectively render the interface substantially "white" when viewed by reflection. Since the reflection density is the result of light being absorbed twice by a given quantity of dye--once when the light enters and a second time when it is reflected back--it will be seen that decolorization of even a few molecules of dye adjacent the interface provides an effect which is so amplified by the optics of reflection that one can substantially lower the reflection density and increase the apparent whiteness of the layer of the processing composition providing the background against which the image is viewed without reducing the transmission density of the "white" layer to any significant extent.
The "decolorizing" layer is provided between the image-receiving layer and the layer of processing composition. This decolorizing layer comprises a substantially nondiffusible agent adapted to decolorize the small concentration of optical filter agent which is present immediately adjacent the interface between the processing composition and the decolorizing layer. This decolorizing is essentially limited to the optical filter agent which is present immediately adjacent the interface between the decolorizing layer and the processing composition. Even though the decolorizing layer is relatively thin, it inhibits diffusion of optical filter agent into the image-receiving layer where it may react with the mordant to form a new species whose color is discharged only at a lower pH; e.g., the new species exhibits a much lower pKa and remains colored until the pH is reduced to a much lower level than otherwise would be required for decolorization. The remaining optical filter agent is discharged or decolorized by a subsequent pH reduction.
In the preferred embodiments of said U.S. Pat. Nos. 4,298,674 and 4,294,907, the decolorizing agent is a polyoxyalkylene polymer and the optical filter agent is a pH-sensitive phthalein dye.
In U.S. Pat. No. 4,367,277 (issued Jan. 4, 1983 to C. K. Chiklis and N. C. Mattucci), there is disclosed and claimed an image-receiving element for diffusion transfer photographic films and processes wherein a layer of unhardened gelatin is coated over the image-receiving layer. The unhardened gelatin layer is adapted to decolorize optical filter agent in the manner aforedescribed, i.e., immediately adjacent the interface between the processing composition and the decolorizing layer.
In the production of image-receiving elements for use in diffusion transfer photographic processing of the aforedescribed type, the provision of a polymeric overcoat on an image-receiving layer oftentimes results in a reduction in the optical clarity of the image-receiving element including such layers. It will be appreciated that the optical clarity of an image-receiving element will be of particular importance where a photosensitive element is photoexposed through the image-receiving element. Inasmuch as image-forming materials, e.g., image-providing dyes, will be diffused through an overcoat layer for mordanting in the image-receiving layer, the inclusion in the overcoat layer of additives or other agents adapted to alleviate haze or other objectionable properties may, in turn, have a detrimental influence on the rate or quality of image formation. In the aforementioned U.S. Pat. No. 4,294,907, there is described the use of a decolorizing overcoat layer comprising a polyether, i.e., a polyoxyalkylene polymer, hydrogen bonded in a complex with another polymer. The overcoat layer provides desired decolorizing effects and is described as sufficiently "hard" to resist rub off and blocking. In an overcoat of the type therein described, the development of haze in the overcoat layer may be observed. It will be appreciated that the provision of such an overcoat layer, having improved clarity and permitting diffusion transfer processing without a loss in photographic performance, will be of particular interest.